Developments made in harnessing solar energy have resulted in three primary types of photovoltaic devices--namely, crystalline solar cells, semi-crystalline solar cells, and amorphous solar cells. Each of these solar cell types produces electricity as a result of what is commonly known in the solar cell field as the photovoltaic effect. This is an inherent feature of a semiconductor region in the photovoltaic device.
In the fabrication of photovoltaic cells, as for example amorphous solar cells, it is necessary to provide means for collecting the current produced from the cell. On the back of the cell, the surface not exposed to solar radiation, an electrically conductive substrate layer, such as stainless steel, is employed to which an electrical connection can be made. On the front surface of the cell, a transparent conductive oxide (TCO) layer and an electrically conductive grid generally are employed which enable electrical current to be collected from the semiconductor region of the cell without unduly obscuring the incidence of solar radiation to the interior of the cell. For a general discussion of the structure and function of amorphous silicon cells and the means of collecting current therefrom, reference may be made to U.S. Pat. No. 4,409,605 to Ovshinsky et al.
Several techniques are presently employed for fabricating the front contact grid of photovoltaic cells. These techniques involve the fabrication of the grid directly onto the transparent conductive layer of the cell. Exemplary of this in situ fabrication of the grids are screen printing with conductive ink, and electroform plating using a mask to form a preselected pattern. These techniques, in addition to being time consuming from a production aspect, present inherent problems in the fabrication of photovoltaic cells. Particularly, the grids thus formed are not readily electrically interconnected when multiple cells are assembled into a photovoltaic array. The small area of the grids, coupled with the material used, for example silver ink for screen printing, precludes favorable soldering or welding of the interconnectors to the grid.
In addition to the foregoing problem, in situ fabrication of grids is more likely to cause shorting of the photovoltaic cell. This occurs because as the semiconductor body and transparent conductive layer are deposited on the substrate, during fabrication of the cell, minute holes, voids or other defects are inevitably formed therein. Though generally not a problem in the operation of the cell, such holes, voids or defects may be filled with the conductive material of the grid at the time the latter is being fabricated on the cell. When this happens, the cell is essentially irrevocably shorted, the transparent conductive layer being shorted to the conductive substrate. Moreover, current generated by other cells in the photovoltaic array is likewise drawn to the short through the grid and interconnects, thereby rendering a considerable area of the array inoperable. Thus, the time, money, and effort expended in fabricating the photovoltaic cell, including deposition of the grid, are lost. These losses result in an increased overall cost in the production of workable photovoltaic cells and arrays.
Efforts have been made to form front contact current collector grids by methods other than in situ fabrication. Exemplary of these efforts is U.S. Pat. No. 4,348,546 to Little wherein a mesh of fine wires is encapsulated between a transparent cover plate and the front surface of the semiconductor strata during fabrication of the solar cell. Use of fine wire as a front contact current collector for solar cells is also discussed in "An Alternative to the `Five Year Research Plan . . . 1984-1988 . . . Photovoltaics: Electricity from Sunlight`. . . U.S. Department of Energy, May 1983", the Grindewald Letter, March 1984 edition (Eleventh in a series), published by Alfred H. Canada, Mammoth Lakes, Calif.
Despite the substantial work and research conducted in the production of photovoltaic cells and arrays, no practical solutions have been developed to alleviate the aforesaid problems in fabricating the front contact current collector grids.